1. Field of the Invention
The present invention relates to control circuitry for controlling a voltage level, and in particular to control circuitry for controlling the voltage output of a DC-DC switched-mode power supply.
2. Discussion of the Related Art
DC switched mode power supplies (SMPS) are known in the art, and are used to generate a DC voltage level which is different from a supply DC voltage level of a circuit. The output voltage may be lower than the input voltage (for example in the case of a buck converter), it may be higher than the input voltage (for example in the case of a boost converter) or it may be either higher or lower (for example in the case of a buck-boost converter). Such circuits comprise a switching transistor, and the output voltage of these circuits is controlled by the duty cycle of the switching transistor.
In order to regulate the duty cycle of the switching transistor, and thus the output voltage, both analog and digital control circuits have been proposed. Digital control circuits have the advantage over analog control circuits of using less chip surface area, and are thus the preferred solution in many applications such as laptops, mobile phones, digital cameras etc.
FIG. 1 of the present application represents FIG. 1 of the publication titled “A Low-Cost Digital Controller for a Switching DC Converter with Improved Voltage Regulation”, M. Murshidul Islam et al., IEEE Power Electronics Letters, Vol. 2, No. 4, and shows an example of a buck converter. As illustrated, the converter comprises a power supply 2 connected to a switch 4, switch 4 being connected to a further switch 6, which is connected to ground, and to a resistance 8 in series with an inductor 10, resistance 8 representing the resistance of the inductor. Inductor 10 is connected to a load 12, and to resistor 14 connected in series with a capacitor 16, capacitor 16 also being connected to ground. Inductor 10 is also connected to the output node 18 of the circuit, and to a feedback path comprising a 4-level comparator 20 and a digital controller 22. Digital controller 22 provides a first output connected to a control input terminal of switch 4, and another output, complementary to the first output, connected to a control input terminal of switch 6.
In operation, during a first phase, switch 4 is turned on while switch 6 is off to provide a current flow through the inductor 10, which stores energy in the inductor. In a second phase, switch 4 is turned off, and switch 6 is turned on such that current continues to flow through inductor 10.
The output voltage can be regulated by controlling switch 4 to be on for longer or shorter periods (and controlling switch 6 in a complementary fashion), in other words by controlling the duty cycle of the control signal to each transistor. The 4-level comparator 20 determines the output voltage with respect to four voltage levels, wherein the desired voltage is between the middle two voltage levels. The digital controller operates as follows:                if the output voltage is below all of the four voltage levels, a ramp up mode is entered in which switch 4 is turned on and remains on until the voltage is higher than the lowest voltage level;        if the output voltage is higher than all of the voltage levels, a discontinuous mode is entered in which switch 4 is turned off to bring the output voltage down rapidly to below the highest voltage level;        if the output voltage is outside the middle two voltage levels, by inside the outer two voltage levels, dither control is used to adjust the duty cycle; and        if the output voltage is between these middle two levels, the duty cycle of the control signals remains unchanged.        
The described control method has several of disadvantages. Firstly, by controlling the circuit to be at the extremes (100% duty cycle, 0% duty cycle) when the voltage is higher or lower than all of the voltage levels, there is a high risk of overshoot as the dithering portions of the circuit will tend to respond too slowly to correct the duty cycle before the voltage level has traversed all of the voltage levels. This results in an unstable control system. Furthermore, such a technique cannot be applied to other types of DC voltage converters, such as boost converters, in which providing 100 percent duty cycle will not result in a maximum voltage increase.